This invention relates to an air heating apparatus for automobiles or for domestic use.
There is known an air heating apparatus, for example, as disclosed in Japanese Patent Application Laid Open No. 55-68413, in which a positive temperature coefficient ceramic heating element (PTC heater) which has a number of apertures therethrough is rotatably arranged in a duct so that when no heating is necessary, the PTC heater is brought into an inoperative position in which it comes out of the air passage of the duct. Otherwise, the PTC heater would be an obstacle to the air passing through the duct. In order to make it possible to rotate the PTC heater between an operative position in which it is exposed to the air flow in the duct and the inoperative position, it is necessary to provide, on a rotating shaft of the PTC heater, electrical slide contacts, such as slip rings, for feeding electrical power to the PTC heater. Furthermore, the PTC heater requires a relatively large electrical current and, accordingly, a large power loss occurs at the slide contacts since the power loss is a function of the square of the current. In addition, the repeated rotational movement of the PTC heater results in wear of the slide contacts.
It is also necessary to use flexible conductors connected to the PTC heater so that the conductors can follow the rotational movement of the PTC heater. However, such flexible conductors tend to break, resulting in a decrease in the service life of the apparatus.
Alternatively, there is also known an air heating apparatus including an immovable PTC heater which is located across and in a duct. However, this type of known apparatus requires the provision of a by-pass passage which bypasses the PTC heater and which has a valve at its inlet end. The valve closes the by-pass passage when heating is necessary, so that air flows through the PTC heater. The valve closes the main duct when no heating is necessary, so that air flows in the by-pass passage. In order to obtain a large amount of hot air, the diameter of the PTC heater which is usually of circular disc shape is required to be increased, which results in an increase of the diameter of the duct.
The primary object of the present invention is to eliminate the above mentioned drawbacks of prior art apparatuses, by providing a hot-air heating apparatus which has a heating element located in and parallel with a duct and control valves which are located at upstream and downstream sides of the heating element to control the amount and direction of air passing therethrough.